Previous work in our laboratory demonstrated that 3-4 year-old children with autism spectrum disorder (ASD) have cerebral enlargement, as well as proportional enlargement of subcortical regions (cerebellum, hippocampi and amygdalae), compared to age-matched children with developmental delay (DD) and typical development (ID). Children with strictly defined autistic disorder (AD) showed enlargement of the amygdalae, in excess of overall cerebral enlargement. From the same groups of children, proton echoplanar spectroscopic imaging (PEPSI) revealed widespread regional decreases in brain neurochemical concentrations and abnormal metabolite relaxation in the ASD children relative to controls, indicative of abnormal celluiar composition. These findings demonstrate abnormal brain developmental processes in autism by 3-4 years of age. In the proposed STAART Center research, we aim to increase our understanding of abnormalities in early brain development by conducting brain imaging research with children with AD at the earliest age it is currently possible to diagnose this disorder. We propose to characterize brain structure and tissue-based neurochemistry in forty-eight 18-24 month-old children with AD and matched comparison groups of 25 children with DD and TD. In the AD group, we will relate brain structural and chemical findings taken at 18-24 months to neurocognitive and symptom measures taken at 18-24 months and later at 4 years of age. Furthermore, based on repeated measures of head circumference obtained retrospectively from birth to 18-24 months, and prospectively from 18-24 month to 4 years, the early developmental course of head growth in autism will be examined.